Cytopathology of Bombyx mori (Lepidoptera: Bombycidae) silk gland caused by multiple nucleopolyhedrovirus

Abstract

ABSTRACT.A multiple nucleopolyhedrovirus previously isolated from infected Bombyx mori L. larvae (BmMNPV) in Paraná State, Brazil, was inoculated into B. mori larvae to examine susceptibility and cytopathology in silk gland cells. The anterior, middle and posterior silk glands were removed from the infected silkworm at different times post-inoculation and processed for cytopathology studies by light and transmission electron microscopy. BmMNPV infection was only detected at 72 h post-inoculation in cells of the middle and posterior silk glands. No sign of infection was found in the anterior silk gland. Cytopathological characteristics were similar to those found in other target tissues; initially, they consisted of nuclear hypertrophy and the formation of virogenic stroma (viroplasm), in which the progeny virions are produced. The virions are then enveloped and occluded in protein crystal structures, the polyhedra. After viral replication, cells undergo lysis and release a great number of BmMNPV polyhedra into the hemocoele. Histopathology showed early infection foci in regions surrounding trachea insertions, demonstrating that trachea is an infection-spreading organ in the insect body. Trachea penetrates the middle and posterior silk gland basal lamina, considered a barrier to virus, facilitating the penetration of budded virus. The anterior silk gland does not have tracheal insertions into the basal lamina, which reduces budded virus infection. Tracheal branches provide a conduit for non-occluded virions (budded virus) to pass through the basal lamina barrier and disseminate BmMNPV in the silkworm silk gland.

Bombyx mori pylorus infection by Alphabaculovirus

“…This cell lysis behavior is characteristic of BmMNPV infections, having been observed in adipose tissues (Brancalhão et al, 2002), the integument (Brancalhão and Ribeiro, 2003), the male reproductive system (Pereira et al, 2008), in the silk gland , and in the cardia (Ribeiro et al, 2009). At this stage, symptom analysis showed leakage of the hemolymph due to frequent integument injuries that were caused by the action of chitinase and cathepsin enzymes encoded by the viral genome (Brancalhão and Ribeiro, 2003;Brancalhão et al, 2009). Thus, hemolymph containing viral polyhedra reaches the environment.…”

“…Subsequently, it becomes larger and takes on a characteristic geometric shape, being present in the nucleus and in the extracellular environment, as displayed by red color-ation ( Figures 1B and 2B) resulting from chemical interactions with the dye (Hamm, 1966). Brancalhão et al (2009) classified these two states as immature and mature, respectively, where the immature forms are not enveloped and the mature forms contain the envelope. The geometrical shape of BmMNPVs was described by Torquato et al (2006b) as a truncated octahedron.…”

“…The geometrical shape of BmMNPVs was described by Torquato et al (2006b) as a truncated octahedron. However, they may also present in other geometric forms, such as cuboids, hexahedral, dodecahedral, triangular, or tetrahedral, which may be the result of genetic variations specific to each viral strain, or even due to polyhedrin mutations (Katsuma et al, 1999;Hong et al, 2000;Ribeiro et al, 2009;Brancalhão et al, 2009;Liang et al, 2013).…”

“…When taken to silkworm rearing rooms, these leaves can infect healthy silkworms, thus restarting the infectious cycle in the new host (Brancalhão et al, 2002;Liang et al, 2013). Thus, it is essential that prophylactic measures for control be adopted as soon as the presence of this virus is noticed, such as the removal of diseased silkworms, as they are a potential focus of the pathogen (Potrich et al, 2007;Brancalhão et al, 2009).…”

Cells of the rectum of Bombyx mori affected by experimental inoculation with Alphabaculovirus

“…BmMNPV is polyorganothropic, and in the larvae of B. mori several targets have been identified such as the fatty body (Brancalhão et al, 2002), integument (Brancalhão and Ribeiro, 2003), central nervous system (Torquato et al, 2006a), testis (Pereira et al, 2008), cardia (Ribeiro et al, 2009a), and the middle and posterior regions of the silk gland (Brancalhão et al, 2009). However, we do not know the behavior of the rectum with regard to this geographic isolate of the virus.…”